1. Field of the Invention
The invention relates generally to the field of integrated circuit (IC) device packages, and more particularly to improved substrate designs in ball grid array (BGA) packages.
2. Related Art
Integrated circuit (IC) dies are typically mounted in or on a package that is configured to be attached to a printed circuit board (PCB). One such type of IC die package is a ball grid array (BGA) package. BGA packages are useful for a number of reasons, including high reliability, and good thermal and electrical characteristics. BGA packages also provide for smaller footprints than many other available package solutions.
A BGA package has an array of solder balls located on a bottom external surface of a package substrate. The solder balls are reflowed to attach the package to the PCB. In a “die-up” BGA package, the IC die is mounted to a top surface of the package substrate. In a die-down BGA package, the IC die is mounted to the bottom surface of the package substrate. Bond wires typically couple signals in the IC die to the substrate. The substrate has internal routing which electrically couples the bond wires to the solder balls on the bottom substrate surface.
A number of BGA package substrate types exist, including ceramic, plastic, and tape (also known as “flex”). In some BGA package types, a stiffener may be attached to the substrate to supply planarity and rigidity to the package. In such packages, the IC die may be mounted to the stiffener instead of the substrate. The stiffener also provides for greater spreading of heat from the IC die. In a die-up configuration, openings in the stiffener may be used to allow the IC die to be bond-wired to the substrate.
In one type of die-down tape-substrate BGA package design, the tape substrate has a rectangular central opening. The substrate is mounted to the bottom surface of a stiffener. The bottom surface of the stiffener has a central cavity that coincides with the rectangular central opening of the substrate. The IC die is mounted to the stiffener, and resides in the cavity formed by the rectangular central opening and the stiffener cavity. A ground ring may surround the IC die in the BGA package. This package configuration suffers from long bond wires that are required to couple the IC die to the substrate. Bond wires coupled to the IC die must cross a portion of the cavity, and the ground ring, to reach signal bond fingers on the substrate.
In high-speed BGA designs, such long bond wires are undesirable. High-speed circuits require short signal lengths with low inductance to operate properly, or to operate at all. Hence, what is needed are BGA packages that provide for short bond wires lengths.